Method of acceleration of the decomposition of sodium-amalgam in decomposers of mercury-cells

ABSTRACT

THE PRESENT INVENTION PROVIDES AN IMPROVED METHOD FOR DECOMPOSING A SODIUM AMALGAM BY ADDING WATER TO SAID AMALGAM WHEREBY AMALGAM IS DECOMPOSED INTO MERCURY, AND SODIUM HYDROXIDE AND HYDROGEN ARE FORMED. THE IMPROVEMENT COMPRISES ADMIXING TUNGSTEN CARBIDE WITH SAID SODIUM AMALGAM AND WATER.

United States Patent ffice 3,649,486 Patented Mar. 14, 1972 U.S. Cl. 204-99 8 Claims ABSTRACT OF THE DISCLOSURE The present invention provides an improved method for decomposing a sodium amalgam by adding water to said amalgam whereby said amalgam is decomposed into mercury, and sodium hydroxide and hydrogen are formed. The improvement comprises admixing tungsten carbide with said sodium amalgam and water.

BACKGROUND OF THE INVENTION In the electrolytic production of chlorine, sodium hydroxide and hydrogen in mercury cells, aqueous sodium chloride solution is electrolytically decomposed between a movable mercury cathode and graphite anodes, sodium amalgam being formed at the cathode and chlorine being separated at the anode. The sodium amalgam is decomposed outside the electrolytic cell in a decomposer into sodium hydroxide and hydrogen. Pieces of graphite which are in intimate contact with the amalgam serve to catalytically accelerate the decomposition process. The mercury which has been largely freed of sodium is returned to the electrolytic cell for further formation of amalgam.

The electrolytic process utilizing movable mercury cathodes is described in C. A. Hampel The Encyclopedia of Electrochemistry, Rheinhold Publishing Corporation, New York 1964, p. 1831? and in I. S. Sconce Chlorine, Its Manufacture, Properties and Uses, Reinhold Publishing Corporation, New York 1962, p. 127ff. It is noted that the decomposition of the sodium amalgam by the addition of water thereto is catalyzed by the presence of nickel or graphite impregnated with solutions of nickel salts in the decomposer and admixed with the sodium amalgam and water. A disadvantage of the foregoing catalysts is that the catalytic activity decreases rapidly due to poisoning by the mercury. It is an object of the present invention to provide an improved method in which the rate of decomposition of the sodium amalgam is increased sub stantially.

SUMMARY OF THE INVENTION The present invention provides an improved method for decomposing a sodium amalgam by adding water to said amalgam whereby said amalgam is decomposed into mercury, and sodium hydroxide and hydrogen are formed. The improvement comprises admixing tungsten carbide with said sodium amalgam and water. The sodium amalgam which is decomposed is preferably obtained from electrolytic mercury cells used in the production of chlorine. These sodium amalgams preferably contain between about 0.2% and 0.3% of sodium.

DETAILED DESCRIPTION OF THE INVENTION The decomposition of the sodium amalgam from the mercury cell is accomplished in the decomposer by adding water thereto. The decomposition reaction is that generally indicated by the following equation wherein x is a small number on the order of about 0.2%-0.3% of the total amalgam:

The sodium concentration in the amalgam may generally vary between about 0.1% and 0.6% (by weight). When there is a higher sodium concentration, the viscosity of the sodium amalgam is increased greatly resulting in its flowability being reduced. Such a concentrated sodium amalgam may decompose under the conditions existing in the mercury cell. This is undesirable and potentially dangerous since the liberated hydrogen may form an explosive mixture with the chlorine produced at the anode. Mercury cells are generally operated to produce a sodium amalgam containing between about 0.2% and 0.3% sodium.

The tungsten carbide is added to the material in the decomposer. This is considered to be the feed sodium amalgam and the feed water although it is recognized that the decomposer may also contain the sodium hydroxide product. The tungsten carbide may be in the form of chunks of tungsten carbide per se or may be in the form of composite articles surfaced with tungsten carbide. Since the catalytic action is believed to be a surface phenomena, the use of composite articles is preferred since they present a greater surface advantage for the quantity of tungsten carbide utilized and also permit the use of a substrate or basis which is surfaced with the tungsten carbide which adds strength to the composite. Tungsten carbide surface coated graphite composites are the preferred form of tungsten carbide to be added to the decomposer. These may be prepared by plasma injection coating of tungsten carbide on graphite to a thickness of about 0.1 mm.

The invention is further illustrated in the following examples:

EXAMPLE 1 Equal quantities in each case of sodium amalgam with 0.3 percent by weight sodium with bare pieces of graphite of well defined surface, or with pieces of graphite coated with tungsten carbide having the same surface, were decomposed in the same decomposer apparatus in the same quantity of 10% caustic soda solution and the times required to develop a given volume of hydrogen were compared. The time for the filling of the volume in the case of bare decomposer graphite was 5 to 6 times the time required in the case of graphite coated with tungsten carbide. The decomposer graphite coated with tungsten carbide did not lose its catalytic activity even upon frequent repetition of the experiments over a long period of time.

EXAMPLE 2 250 grams of sodium amalgam containing 0.3% by weight of sodium was decomposed in a 500 cubic centimeter Erlenmeyer flask to which was connected a gas take-ofl tube. Two tests were run. In two tests pieces of graphite (30 x 30 x 10 mm.) were added and the sodium amalgam decomposed to produce 50 ml. of sodium hydroxide. The time necessary to accumulate 50 ml. of hydrogen was noted. In the first test this was accomplished in 403 seconds and in the second tests 466 seconds.

A duplicate set of tests were carried out except that the composite tungsten carbide surfaced graphite was used in place of the graphite in the comparative tests. The time necessary to collect the 50 ml. of hydrogen was measured as 73 seconds in one test and 103 seconds in a second test.

EXAMPLE 3 Into a horizontal decomposer of a 50 kilo-amp. mercury cell, 24 graphite pieces surfaced with tungsten carbide to a thickness of 0.1 mm. (total dimension being 500 x x 60 mm.) were fed, and water was also fed at a speed sufiicient to produce a caustic solution having average content of 50% sodium hydroxide. In another identical horizontal decomposer, 51 graphite pieces having the same aforementioned dimensions were added and the same conditions maintained. The sodium amalgam feed to both decomposers was an average of between 0.2% and 0.3%. Both decomposers were recorded and compared during a 6 month period.

The decomposer having the tungsten carbide graphite composites produced a mercury product containing an average of 0.01% sodium. The sodium content of the decomposer containing only graphite, produced a mercury product containing an average of 0.08% sodium. As a result of the foregoing, the chlorine gas produced from the mercury cell to which the 0.08% sodium amalgam was recycled produced chlorine gas containing an average hydrogen content of 0.4% (by volume) while the hydrogen content of the chlorine produced in the cell to which the 0.01% sodium amalgam was recycled was only 0.1% (by volume).

During the 6-month period, the tungsten carbidegraphite composites maintained their shape and essential size, whereas the graphite pieces after the 6 month period exhibited an average volume shrinkage of about 20%.

The foregoing examples illustrated the major advantages flowing from the use of the tungsten carbide catalyst. The decomposition rate is substantially increased, e.g., x6x. There does not appear to be any poisoning of the catalyst during the process. This results in major advantages in the economy of operating the decomposer cells and also in major product advantages and operating advantages in the electrolytic mercury cell to which the decomposer is an adjunct.

While I have disclosed several embodiments of the present invention, it is to be understood that these embodiments are given by example only and not in a limiting sense.

What is claimed is:

1. In the method of decomposing a sodium amalgam by adding water to said amalgam whereby said amalgam is decomposed into mercury, and sodium hydroxide and hydrogen are formed, the improvement comprising admixing tungsten carbide with said sodium amalgam and water.

2. The process of claim 1 wherein said sodium amalgam contains between about 0.1% and 0.6% sodium.

3. The process of claim 2 wherein said tungsten carbide is in the form of a composite article having tungsten carbide on the surface and a graphite base.

4. The process of claim 1 wherein said sodium amalgam contains between about 0.2% and 0.3% sodium.

5. The process of claim 4 wherein said tungsten carbide is in the form of a composite article having tungsten carbide on the surface and a graphite base.

6. In the process of decomposing the sodium amalgam product of an electrolytic mercury cell in the process for producing chlorine, sodium hydroxide, and hydrogen wherein the sodium amalgam, containing between about 0.2% and 0.3% by weight sodium, is transferred to a decomposer and water added thereto to decompose said sodium amalgam to produce sodium hydroxide, hydrogen, and mercury and wherein the mercury is recycled to the electrolytic mercury cell, the improvement comprising adding tungsten carbide to the sodium amalgam and water in the decomposer.

7. The process of claim 6 wherein the tungsten carbide is in the form of a surface deposit on a support material.

8. The process of claim 6 wherein the tungsten carbide is in the form of a composite article having tungsten carbide on the surface and a graphite base.

References Cited UNITED STATES PATENTS 1,863,254 6/1932 Polin 204-250 X 2,597,545 5/1952 Taylor 204-250 X 3,068,157 12/1962 Vielstich et al. 204-248 X 3,497,432 2/1970 Hauck 204-99 JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner US. Cl. X.R. 204-128, 129, 220 

